Generally, a fuel cell is an energy conversion device which converts chemical energy into electric energy by the electrochemical reactions between a hydrogen-containing compound, for example hydrogen gas, is a fuel and oxygen or air which is an oxidizing agent. The reaction typically takes place across a membrane. The polymer electrolyte membrane fuel cell membrane advantageously comprises a polymer as an electrolyte. A typical polymer electrolyte fuel cell is constructed by assembling a membrane-electrode assembly, gaskets, and separators.
A configuration of a conventional polymer electrolyte membrane fuel cell is as follows. Anode and cathode are placed on either side of a hydrogen ion-conductive polymer membrane. That is, the membrane typically has a first and second sides, and includes electrodes, typically an anode and cathode, one on each side thereof. The electrode for the polymer electrolyte membrane fuel cell may include, for example, carbon supported platinum or platinum-ruthenium powder for the anode and carbon supported platinum powder for the cathode.
Generally the anode and the cathode are manufactured independently by spraying catalyst ink prepared by mixing Pt or Pt/Ru catalyst powders and a polymer electrolyte ionomer on hydrated carbon papers. The electrodes for a polymer electrolyte membrane fuel cell can be manufactured by thinly coating a porous and water proof carbon paper or film with a catalyst-containing ink which is a mixture of the catalyst powder and advantageously a polymer electrolyte solution, that is, an ionomer such as Nafion solution. Typically, a spray coating, a filtration, or a screen printing method is used for coating the carbon paper with the catalyst ink.
Then, a membrane-electrode assembly may be manufactured by putting an electrolyte membrane between the anode and the cathode manufactured in this way, and hot-pressing them at a temperature above the glass transition temperature of the electrolyte while applying a predetermined pressure. The membrane-electrode assembly is a configurational element of the fuel cell which may optionally include one or more additional layers therein.
To construct a single cell, gaskets are used to achieve air tightness between the membrane-electrode assembly and separators. Operation of the fuel cell at normal, i.e., non-freezing, temperature is started by supplying hydrogen and air passing through a humidifying device to the unit cell. The supplied hydrogen and air generate electric current and water by the electrochemical reactions. The reaction formulae at the anode and cathode of the polymer electrolyte membrane fuel cell are as follows.
Anode reaction: H2→2H++2e
Cathode reaction: 1/2O2+2H+→H2O
FIG. 1 is a schematic diagram of a unit cell. During normal operation the water, supplied through the humidified gas and/or generated by the electrochemical reaction, exist in various parts of the unit cell, including the cathode gas flow channel 2, the cathode catalytic layer 5, the polymer electrolyte 6, the anode catalytic layer 7, and the anode gas flow channel 10. Reference numeral 1 and 9 notes a first separator and second separator. Reference numeral 3 notes gaskets. Reference numeral 4 is a cathode carbon paper. Reference numeral 8 represents an anode carbon paper.
When operation of an operating polymer electrolyte membrane fuel cell is stopped and the external temperature of the polymer electrolyte membrane fuel cell is below the freezing point of water in the winter, the water contained in the gas can condense, and the liquid water, i.e., that water supplied by the humidified gas and that water generated by the electrochemical reaction, can freeze.
In such a unit cell the freezing of liquid water causes membrane-electrode assembly (5, 6, and 7) to be damaged due to a volume expansion of the water, and as a result performance of the fuel cell is degraded.
To address the problem, U.S. Pat. No. 5,798,186, the contents of which is incorporated herein by reference, describes a method for removing water in the passages and the membrane-electrode assembly by flowing inert non-humidified gas for a long time. However, this conventional method has a disadvantage in that it needs to flow much inert non-humidified gas for a long time in order to sufficiently remove the internal condensed water.